Wind farms are commonly located in remote areas to take advantage of good wind conditions. This may require long transmission lines to connect the wind farms to the rest of the power grid. Because of the long transmission lines and extreme conditions in the remote areas the wind farms may have to be designed to withstand relatively high levels of fault conditions, this is also necessary in order to ensure good connectivity with the grid. Overvoltage is one such fault condition which should be handled properly.
In W. Sweet, “Danish Wind Turbines Take Unfortunate Turn”, IEEE Spectrum, vol. 41, no. 11, pp. 30, 2004 it was reported that on the west coast of Denmark an offshore wind farm called Horns Rev 1 connected by an undersea cable experienced temporary overvoltages (TOV) as high as 2 p.u. when the main-circuit breaker tripped at the on-land connection point and left the wind farm in isolated operation with the cable and the wind farm transformer.
In W. Wiechowski, J. C. Hygebjerg and P. Børre Eriksen,—Higher Frequency Performance of AC Cable Connections of Offshore. 7th int. Conf. on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms, pp. 211-217, 2008 it was reported the Danish transmission system operator, Energinet.dk, performed investigations of such overvoltage in connection with the planning of the new offshore wind farm Horns Rev 2. These investigations have shown that the overvoltage levels are influenced by many parameters, including operational characteristics of the wind turbine generator (WTG) prior to the disconnection, protection systems, control and the accuracy of the representation of the cable and the transformers in the relevant frequency range.
In each country the grid code specifies what fault conditions a wind farm must be designed to withstand before tripping. This directly affects the design of the WTG. As shown in FIG. 1, the dotted curve 1 corresponds to Australia where the temporary overvoltage up to 1.6 pu can be experienced. For Canada the overvoltage in FIG. 1 could be up to 2 pu for a short duration. In FIG. 1 if we do not consider Canada and Australia, the solid curve 2 covers temporary overvoltage grid requirement for all other countries.
Wind farm owners and wind turbine manufacturers may wish to protect the power electronics and power components in converter systems of wind turbines from damage caused by the overvoltage. The easiest solution to protect the turbines is by disconnection from the grid. However, transmission system operators may want to maintain grid connectivity of wind farm in case of overvoltage. Thus the relevant grid code may require the turbine to stay connected to the grid during such overvoltage events.
International patent publication number 03/058789 discloses a number of current limiters in the form of series impedances in parallel with power electronic switches. The current limiters operate independently for the respective phase when the line current in that phase is above a predetermined limit. During the fault condition, the power control is disabled, enabling voltage control with a fast current controller. In this publication, the series resistors seem to absorb most of the active power in the fault conditions which makes them bulky. This problem appears to be more serious with growing size of the WTGs.